Power boosters have been used in vehicle brake systems, as well as in other systems, for many years. One of the more common power brake boosters is powered by vacuum generated by the vehicle engine or a vacuum pump. Such boosters have been using a rubber-like reaction disc which is contained in a reaction disc chamber in the booster power wall. The booster has an output member which has one end forming a wall of the reaction disc chamber, the output member being axially movable in the chamber. The other end of the reaction disc chamber is defined by an annular shoulder forming an outer diameter portion of the wall. The shoulder is a part of the booster power wall. A reaction piston is reciprocally received in the passage passing through the annular shoulder and forms the other part of the booster power wall. A reaction disc is located in the chamber. The disc is generally puck-shaped, having a circumferentially extending peripheral wall and oppositely disposed side walls. The peripheral wall has a diameter only slightly less than the diameter of the chamber. One of the side walls is engaged or engageable by the portion of the output member forming one chamber end wall. The other side wall is engaged or engageable on an annular outer surface thereof by the annular shoulder, and the inner surface of the other side wall is engaged or engageable by the end of the reaction piston forming the inner part of the other chamber end wall.
When the booster is actuated, the output member transmits reaction force to the one side wall of the reaction disc, and this force is resisted by the annular shoulder and the reaction piston as they engage the other side wall of the reaction disc. This force tends to compress the disc. One part of it is transmitted to the annular shoulder and the other part of it is transmitted to the end of the reaction piston. The ratio of these parts is determined by the ratio of the area of the shoulder to the area of the reaction piston end. Due to the compression of the disc, the disc peripheral diameter is increased and the disc may engage the cylindrical wall of the disc chamber, somewhat modifying the ratio of reaction forces. This compression also tends to cause some extrusion of the disc in the small annular space created by the smaller diameter of the output member in relation to the diameter of the reaction disc chamber.
Reaction discs in current use, and in use for many years past, have had one durometer or hardness value throughout the disc. In order to obtain good reaction force definition, this durometer or hardness value has resulted in reaction discs which are somewhat softer than they might otherwise be, and they tend to so expand and be extruded.